Fluid is commonly pumped though tubing inserted into a well to drill or to provide intervention services, such as stimulation or milling of obstructions. Means for pulsing this flow of fluid have been developed for a variety of applications, including mud pulse telemetry, well stimulation, enhanced drilling, and to extend the lateral range of drilling motors or other well intervention tools. For example, commonly assigned U.S. Pat. No. 6,237,701 and U.S. Pat. No. 7,139,219 disclose hydraulic impulse generators incorporating self-piloted poppet valves designed to periodically at least partially interrupt the flow of fluid at the bottom end of the tubing. At least partially interrupting the flow of fluid in this manner leads to an increase in pressure upstream of the valve and a decrease in pressure downstream of the valve.
Pressure pulsations in the tubing upstream of the bottomhole assembly (BHA) have a variety of beneficial effects. The pulsations can improve the performance of rotary drilling by applying a cyclical mechanical load on the bit and cyclic pressure load on the material being cut. In combination, these loads can enhance cutting. In addition, the pulsating vibrations induced by these tools in the tubing can reduce the friction required to feed the tubing into long deviated wells.
The valve also generates pressure fluctuations or pulses in the wellbore near the tool. These pressure pulses can enhance chemical placement in the formation and enhance the production of formation fluids such as oil or gas. In addition, these pulses can be employed to generate a signal that can be used for seismic processing.
The valve designs disclosed in U.S. Pat. Nos. 6,237,701 and 7,139,219 generate a relatively short pressure pulse, which limits both pulse energy and the effectiveness of the pressure pulse. Commonly assigned U.S. patent application Ser. No. 12/957,049 describes an improved apparatus that limits the pressure differential causing the valve to shift from the open to the closed position and incorporates flow restrictions that further limit the shift rate of the valve between the open and closed positions. The apparatus incorporates a spool valve with clearance seals between sliding valve parts. These clearance seals are wear areas, so that the clearance area at the seals may vary during the time that the valve is in service. When the valve is closed, a critical clearance seal area is subject to high differential pressure. Leakage across this clearance seal gap increases the shift speed and reduces the time that the valve stays closed. Close control of this timing is critical for effective operation of the valve. The clearance tolerance range required for acceptable operation is small, and there can be substantial variations in valve performance if the tolerance range is not met. Increased clearance causing increased fluid leakage through the seals is associated with reduced pulse amplitude and duration. Accordingly, it would be desirable to develop a seal that limits leakage in this area of the valve and provides longer pulses, resulting in more uniform pressure profiles.
Further, it would be desirable to increase the amplitude and duration of pulses produced by a hydraulic pulse valve. It would also be desirable to reduce the variability in the pulse profile caused by clearance variations and wear and to provide a reliable, debris-resistant means for adjusting the timing of the valve, i.e., the time required for the valve to move between the open and closed states.